Warewasher machines fall into two generally distinct but somewhat overlapping categories, namely, commercial (restaurant, institutional or other public facility) warewashers and domestic (home) warewashers.
The cleaning efficacy and sanitization results of domestic warewashers are left to the manufacturers of such products. Most of such warewashers are designed to perform washing with an input water temperature of 140.degree. F. This is the basic temperature at which domestic warewashing detergents are formulated to perform most effectively. The warewashers are filled with clean water and drained between three and six times for each wash cycle, depending on whether the load of dishes is lightly or heavily soiled. The operator may select any of the several different cycles he or she may wish to use. One or two of those fills will normally have detergent added to assist in stripping the soil from the dishes by means of a relatively high velocity rotating spray of pumped wash solution recirculated from a sump at the bottom of the warewasher. After washing, rinsing is accomplished by filling the sump with fresh water, recirculating it, draining the water, refilling the sump one or two additional times and repeating the rinsing operation. Such warewashers have relatively long time cycles (e.g. 50 to 70 minutes) and are used once a day on the average.
Because domestic warewashers are used by consumers primarily for their own families, the competitive marketplace provides the necessary incentive to manufacturers to design and build machines which do an effective job of washing and sanitizing.
Commercial warewashing is an entirely different matter. It involves highly productive machines which have fixed, short washing and rinsing cycles, measured in seconds rather than minutes. The end responsibility for washing dishes commercially is left to a businessman dealing with members of the public whose health may be at stake when using dishes washed under that businessman's control. Because of this, there came into existence in the late 1940's an organization called the National Sanitation Foundation (N.S.F.). One of its functions is to provide minimum standards to assure that dishware washed in commercial warewashers is in fact sanitized through bactericidal treatment considered by health authorities to be effective. This is achieved essentially in two different ways according to N.S.F. standards: (1) by high temperature machines which utilize a final rinse minimum time and volume and minimum temperature of 180.degree. F. to assure thermal death of bacteria, or (2) by so-called low temperature machines which rinse with water at a minimum temperature of 120.degree. F., inadequate to sanitize by itself, but which contains an effective germicidal chemical such as sodium hypochlorite (NaOCl). The chemical sanitizing additive must be proportioned in the rinse water at a minimum of 50 parts per million of available chlorine. Available chlorine can be defined as chlorine available to sanitize.
Any proportion less, or anything less than 180.degree. F. final rinse water in a high temperature machine, is regarded by N.S.F. as not providing a proper safety margin for sanitization. In high temperature machines, N.S.F. has scientifically established a cumulative heat factor for a total or complete wash and rinse cycle. The heat factor is measured in "heat unit equivalents" (HUE, later defined) per second of time, which, cumulated, must reach a minimum total of 3600 HUE to be considered effective.
While N.S.F. standards are theoretically voluntary, public health and sanitation officials in the U.S. are believed to rely heavily on them. A manufacturer is permitted to place an N.S.F. label on the equipment to show that its design, manufacture and operation meet all of the minimum N.S.F. standards for that particular type of equipment. Many sanitation officials will not permit installation or use of commercial warewashers within their jurisdiction unless they have N.S.F. labels, indicating that they are "listed" as being recognized by N.S.F. In effect, N.S.F. standards are so well accepted that very few commercial warewashers are sold in the U.S. without N.S.F. listing.
In both the domestic and commercial warewasher field it has become fairly well recognized since the advent of the so-called "energy crisis" of the mid 1970's that an important way to reduce the cost and consumption of energy would be to decrease the volume of hot water used to wash dishes. For several years, domestic warewasher manufacturers have been working diligently to reduce water consumption to a minimum which would still provide satisfactory warewashing in terms of cleanliness in order to remain competitive. It can be said that, in the domestic warewasher field, it has been and continues to be conventional practice to seek to reduce energy consumption by decreasing the use of hot water.
Reducing energy consumption and cost in a commercial warewasher is quite another matter, particularly in view of the minimum standards established by N.S.F. A variety of different types of commercial units exists. In machines using the hot water sanitization principle, the N.S.F. standard minimum temperature is 180.degree. F. for the final rinse water, as noted. In addition minimum water volumes are specified according to the size of dishrack handled by the machine. Since a minimum rinse temperature of 120.degree. F. is acceptable in a chemical sanitizing low temperature machine, in terms of effective sanitization, the use of the sanitizer is equivalent to an energy saving of 60.degree. F. reduction in the final rinse temperature. Offsetting the energy savings of chemical sanitizing machines, however, is an increase in the cost of chemicals and also the initial and servicing cost of equipment for dispensing the bleach and controlling water fill in the proper proportions for each warewasher cycle.
Faced with the problem of saving energy and its cost, and further faced with the 180.degree. F. temperature and water volume requirements of high temperature machines, the primary industry solution in the past half dozen years or so has been to emphasize low temperature machines with chemical additives. Sales of low temperature machines have increased substantially in recent times, partially at the expense of sales of energy consuming high temperature machines. But such low temperature machines are not without fault, unfortunately. A prime disadvantage is that the lower temperature of the foodware items at the time of removal from the warewasher makes it considerably more difficult for them to air dry than when rinsed at 180.degree.. Greater heat in the items from a hot water sanitizing machine tends to drive off remaining moisture much faster. In some instances the foodware from a low temperature machine may have to be reused immediately, while still partly wet. Ordinarily the items shouldn't be towelled, because of the potential of defeating the sanitization purpose. Many public health codes strictly forbid towel drying of dishes. In some restaurants, additional costly space and tabling in the kitchens have been provided to allow dishes washed in low temperature machines a greater period of time for drying. Nevertheless, many low temperature machines continue to be sold because of the reduction in energy costs as compared to presently existing high temperature machines. This is true even though some low temperature machines have greater overall annual operating costs due to greater use of expensive chemicals, water and time to wash items.
Chemical sanitizing low temperature warewashers have been known for many years. Originally they were intended for application where high temperatures were unacceptable, such as for a glass washer installed under a beverage bar. For general warewashing, however, they have been more widely used since the start of the energy crisis of the mid and late 1970's.
Earlier work on low temperature machines is exemplified by U.S. Pat. Nos. 2,592,884; 2,592,885; 2,592,886; 3,044,092; 3,146,718 and 3,370,597. These attempted to utilize the developed concepts of high temperature machines in conjunction with metering sodium hypochlorite directly into the fresh water lines used for final rinsing. While these machines performed satisfactorily for short periods of time, they were not really successful, particularly in hard water situations. Sodium hypochlorite tends to precipitate minerals, particularly calcium, magnesium and iron, from the fresh water at the point of introduction of the extremely small quantity of sodium hypochlorite metered into the water. This point in some instances was a tiny orifice in a venturi where the hypochlorite enters the fresh water line, and in others, it was the exit of a dosing system for hydraulically pumping the minute quantity of hypochlorite through a small orifice into the water line. Once such small orifices were affected by mineral build-up, metering became inaccurate or was cut off and required either replacement or frequent cleaning. It is doubted whether such systems using small orifices are in successful commercial existence in the U.S. today in low temperature warewashers.
At least as early as the 1940's, another form of chemical sanitizing low temperature warewasher became known, initially on a relatively small scale. It is referred to in the commercial warewasher trade as a "fill and dump" machine. It is of the general design and operation shown in U.S. Pat. No. 3,903,909.
In such machines the rinse water is recirculated through the same screen, pump, pipes and wash arms used by the dirty wash water, thus reducing cleanliness in the process. Injection of sanitizer directly into the sump of such warewashers is typically done by peristaltic or pressure pumps.
In addition to the fill and dump machines, several manufacturers (including the assignee of the present invention) have introduced a chemical sanitizing low temperature commercial warewasher which utilizes a fresh water rinse independent of the washing system, but mixes the fresh water and sodium hypochlorite in an auxiliary holding tank in their proper predetermined proportions prior to use by spraying through a rinse system dedicated solely to fresh rinse solution. This concept is shown in U.S. Pat. No. 4,147,558.
The assignee of this invention had a research program in progress some years ago to design a fill and dump machine of the type shown in aforementioned U.S. Pat. No. 3,903,909. The cleaning results observed in that program were deemed unsatisfactory, however, because the soiled wash water and the fresh rinse water were both circulated through the same pumps, strainers and piping, frequently resulting in soil carryover from the wash solution to the rinse water and redepositing soil specks on the foodware. While such foodware items may be completely sanitized according to N.S.F. standards and test methods, their apparent lack of cleanliness often gives restaurant customers the impression that the items are unsanitary. The consuming public frequently associates soil on ware with lack of sanitation.
Thus, although chemical sanitizing warewashers have achieved a niche in the marketplace, the drying problem has been and continues to be a concern. Obviously, drying of the type used in domestic warewashers (where perhaps only one load of dishes may be washed per day) is not at all acceptable in a commercial warewasher environment, where one load of dishes may have to be washed every minute or minute and a half during a main meal period. Even if such drying were made possible, the energy cost of that drying would most likely defeat the very purpose of low temperature warewashing. Further, since N.S.F. specifies differing minimum water volume usage for the various types and sizes of warewashers in the final rinse period of high temperature machines, and since the machines of most manufacturers already appear to be operating right at, or very near, those minimum water volumes, reduction in hot water volume usage below the N.S.F. standard has not appeared to be a feasible alternative in improvement of high temperature machine efficiency. In order to exhibit the N.S.F. seal on the unit, the N.S.F. water volume minimum has seemingly been an untouchable standard, and it thus appears that no one has searched for a way to modify it. Instead, the industry seems to have directed innovative efforts in other directions.
National Sanitation Foundation Standard No. 3 for Commercial Spray Type Dishwashing Machines, includes:
Section 6.0.3 Heat Unit Equivalents: Those commercial spray type warewashing machines relying on heat for sanitization shall, when installed and operated in accordance with the manufacturer's instructions, produce at least 3600 heat unit equivalents when evaluated in accordance with Appendix A "Heat Sanitization." PA1 Appendix A States: PA1 HEAT SANITIZATION: NSF will use as a guide, Methods of Measuring Heat Unit Equivalents, by J. L. Brown. This document is available from NSF, NSF Building, Ann Arbor, Mich. 48105
A general definition of Heat Unit Eguivalent (HUE) is the amount of heat applied to a foodware surface during exposure to heat within a warewasher, and this unit of heat relates to the degree of bacterial destruction achieved. In actuality, the cumulative heat factor adopted by N.S.F. includes a comfortable safety margin. High temperature commercial warewashers have been required to produce a measurable cumulative heat factor of 3600 HUE for a complete wash and rinse cycle of the dish machine. HUE value is determined by first measuring the Fahrenheit temperature of a dish surface at each single second of time. For each second at 143.degree. F., or above, a different HUE value is obtained. The HUE value is logarithmically related to arithmetic increase in foodware surface temperature. For example, at 143.degree. F. (61.66.degree. C.), the HUE value is 1.0, at 153.degree. F. (67.22.degree. C.) HUE value is 14.3, and at 163.degree. F. (72.77.degree. C.), the HUE value is 203.9. The cumulative heat factor is arrived at by adding all the temperatures for each second of a complete cycle, start to finish. N.S.F., in specifying a given minimum volume of water at a minimum wash temperature of 150.degree. F. for stationary rack machines and 160.degree. F. for conveyor machines for a given minimum time period, and then doing likewise for rinsing with a minimum 180.degree. F. water, has in essence said that a cumulative heat factor or level of 3600 HUE is achieved when those minimums are met.